英语语法问题十分费解Our class room i s lighter than 【 that】 the next d

Our class room i s lighter than 【 that】 the next door.
本题有人说在that
后加of
有人说改为the one
有说改为that one
到底改为什么?
问题补充：
Our class room i s lighter than 【 that】 next door.
哦题干没有the是next door
答：这是一道有问题的题目.
如果意思是：我们教室比隔壁的明亮.则：
Our classroom is brighter than the one next to ours
可惜得很,楼上所有的人都没有注意到形容词的误用.lighter显然为brighter之误用,因为light作形容词是轻的,淡的,用来修饰classroom,头一次见到.
而next-door是作前置定语用的：in or being the next building,exp.in a row:next-door neighbours
next door adv.要后置：the neighbours next door= the people living next door.
next door不作名词和,如作名词 The next door则为‘下一扇门’,比较对象不对称.
所以我认为要避开隔壁（next door）这个语言定式,象我那样改比较好：the one =the classroom next to ours.
或更简洁的：
Our classroom is brighter than the next one.
不过,our与the next也不对称.应our与their对称.